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Evaluation of Hygroscopic Cloud Seeding Flares

Roelof T. Bruintjes, Vidal Salazar, Trudi A. Semeniuk, Peter Buseck, Daniel W. Breed, Jim Gunkelman


     A test facility has been designed to provide a reproducible environment for combustion of flares and measurement of the resultant particles. The facility provides a simulation of the environment that a flare would encounter from an aircraft.

     The facility was used to evaluate the concentrations, sizes and chemistry of many flares with different chemical formulations. Earlier studies of particle sizes produced by the South African flares indicated that a cooler burning flare could potentially produce larger particles. However, initial field studies in 2002 did not support this hypothesis. Small particles were dominant at both the beginning and end of the flare burn, when the burn was coolest,. In addition, comparison of the Ice Crystal Engineering (ICE) 65% and 70% potassium perchlorate- containing flares indicates that the 70% flare produced larger particles, despite having more oxygen present to yield a hotter burn.

     The main characteristic of the production of particles by the flare during a burn is that the variations in the total concentration of the particles are small. However, the concentrations of larger particles (>1μm diameter) varies substantially during the individual flare burns and from flare to flare. The latter variations seems to be due to the manufacturing process, including variations in the chemical composition (mesh size, purity, reactions within chemical species, etc) and the cover tube the flare material is compressed in. These changes in the particle size need to be explored further.

     Comparisons among the particle spectra from the ICE 65, 70 and 80% KClO4 hygroscopic flares showed that an increase in the amount of the hygroscopic salt (KClO4) seem to slightly increase the number of larger particles. The larger proportion of the oxidizing salt gives a higher burning temperature, shifting the final size distribution towards larger particle sizes. Based on Scanning and Transmission Electron Microscopy (SEM and TEM) analyses of the ICE 70% flare it seems that the larger particles produced by the flares are composed of aggregate mixtures of KCl and Ca(Cl)2 and are not single particles. Aggregation or coagulation of particles is thus the primary mechanism producing larger particles.

     Based on parcel modeling studies, the new ICE 70% flare produces substantially more drizzle drops at shorter times than the South African flare. After 1 minute, the new ICE flare initiates drizzle and concentrations of drizzle water reach a maximum, when the South African flare just starts producing drizzle size drops. In addition, the drizzle results in a more effective coalescence process, forming rain. Once drizzle is formed, the transformation to rainwater proceeds faster than with the original South African flares. After approximately 10 minutes, the new ICE flare produces nearly two orders of magnitude more drizzle water than the South African flare.

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